A standard left ventriculomyotomy and myetomy (LVMM) has been performed for relief of left ventricular outflow tract obstrucvtion secondary to idiopathic hypertrophic subaortic stenosis (IHSS) in 412 patients. This report summarizes 82 patients undergoing an IVM&M for resting and provokable gradients since January 1982. An attempt has been made to define criteria for choice of operation, LVM&M vs mitral valve replacement (MVR) based upon septal thickness, distribtuion of hypertropny, level of systolic anterior motion (SAM) contact of septum, and conmitant coronary artery diease. Intraoperative 2-D and M-mode echos have been performed on a number of these patients providing precise data utilized intraoperatively. Patients with concomitant CAD have a greater risk for an iatrogenic VSD creation which may be avoided by a modified LVM&M or MVR. Operative mortality is 8.5% and late mortality 5.3%. Hemodynamic data are presented based on preoperative resting gradients less than 50 mm Hg and greater than 50 mm HG and postoperative resting aNd provokable gradients are compared between these groups. Postoperative hemodynamic studies reveal good relief of resting gradient in most patients but significant provokable gradients remain in some patients. Reoperation has been performed in some patients with persistent symptoms and gradients. Medical therapy is continued in patients with signfiicant gradients regardless of symptomatic status.